Conventional methods of mounting electronic module packages ("modules") on printed circuit boards have been achieved with pin grid arrays (PGAs). Typical PGAs are used for mounting devices with large numbers of pins into printed circuit boards.
The prior art PGAs generally included a module having rigid pins extending through the plated through holes in the printed circuit boards. These pins are mounted in these plated through holes with conventional solder, which forms a solder fillet having a very high angle.
This PGA exhibits several drawbacks. A major drawback is the limitation on the lead density, as the plated through holes must be of a sufficiently large diameter (approximately 0.102 cm) to receive the large diameter pins. As a result, these openings occupy space on the printed circuit board which limits the number of possible electrical (pin-plated through hole) connections, or circuit density, as commonly referred to in the art. Additionally, solder of the solder joint, which attaches and electrically connects the pins to their respective plated through holes, forms solder fillets, that extend a substantial distance upward along the pins at high angles. This structure produces high stress in the rigid pins and substantially negates the ability of the pins to bend, in response to thermal and other stresses. As a result, the pins and solder joints are subjected to high sheer strains, increasing the likelihood of breakage of the connection between the chip and the printed circuit board, potentially shortening the useful life of the electronic device employing the printed circuit board.
Additional prior art PGAs, that improve on those discussed above, include printed circuit boards having enlarged openings at the ends of the plated non-through holes, on the top side of the printed circuit boards. These enlarged openings receive rigid pins. The pins are soldered in the openings leaving solder fillets of very high angles. The bottom side of the printed circuit board includes multiple terminals designed to accommodate wiring to compensate for the space lost by the large pin and hole diameters (approximately 1 mm) on the top (pin-receiving) side of the printed circuit board.
This device exhibits the drawbacks discussed above with respect to the high stress and breakage problems associated with the solder fillets. Additionally the large pin and hole diameters restrict the wiring to a single (bottom) side, thus limiting the amount of possible electrical connections.
Surface mount technology has also been employed with printed circuit boards. Surface mount technology has gained acceptance as the preferred means of joining electronic devices together, particularly in high-end computers. As compared to more traditional pin connector methods, like the above discussed PGAs, where a pin mounted to the backside of a ceramic module is thrust through a hole in the board, twice the number of modules can be placed at the same board area. Other advantages such as smaller component sizes, greater I/O densities, lower electrical resistance, decreased costs, and shorter signal paths have prompted the industry migration to surface mount technology.
Conventional surface mount technology involves Column Grid Arrays (CGAs) or Ball Grid Arrays (BGAs), which mount to pads, which are in turn connected to vias. CGAs are integrated circuit chips or modules which have a rectangular matrix of contacts for a substrate, such as a printed circuit board or the like. The contacts are cylindrical columns of solder each having one end bonded to the chip or carrier module. BGAs differ from CGAs in that they include approximately spherical shaped solder balls instead of the cylindrical columns. An example of a CGA using conventional surface mount technology is illustrated in FIGS. 1 and 2 labeled "PRIOR ART".
FIG. 1 shows the CGA surface mount pad arrangement on the printed circuit board 20 with the module 22 (FIG. 2) removed. The top surface of the printed circuit board 20 includes a via (plated through hole) 24 which receives a solder column 26 (FIG. 2) from the module 22 (FIG. 2). A circuit wire 28, extending from the via 24, electrically connects the via 24 to a surface mount pad 30. This connection is commonly referred to as a "dog bone" pattern based on its shape.
FIG. 2 shows the mounting of the module 22 to the printed circuit board 20 in detail. Solder columns 26 extend from the module 22, and are connected to their respective surface mount pads 30, by solder joints 32 having solder fillets 34 against the solder columns 26. Through the circuit wires 28 (FIG. 1), electrical contacts are made with the vias 24.
This surface mount technology improved on the PGA technology. By using flexible solder columns with lower angled solder fillets, the printed circuit boards have longer usable lives. The chances for stress decrease as the flexible solder columns and solder fillets are able to accommodate the thermal expansion and contraction associated with this CGA technology. However, this prior art CGA exhibits a major drawback in that the "dog bones" and mounting pads occupy significant surface area of the printed circuit board, limiting circuit density. To bring the "dog bone" arrangements closer together to increase density is problematic, in that unwanted electrical shorts may occur should the components of these "dog bone" patterns accidentally come into contact with each other.